1. Field of the Invention
The present invention relates to an infrared observing system, and more particularly to a system for representing a connected high frequency component of an infrared image signal by the completion of a flat frequency transfer characteristic as a whole. Thus, the possibility of a quantitative system design leads to high resolution and low cost.
2. Description of the Related Art
In general, when a light detection output is represented by an infrared-ray video signal, it is necessary that a frequency transfer characteristic of the infrared-ray video signal be made flat all over the system so that the display unit representation is correct temperature information that can be easily seen.
In a prior art design technique for infrared-ray observing equipment, an MTF (modulation transfer function) of frequency transfer characteristics in relation to an optical and scanning system does not reduce the frequency characteristics in a high frequency region (viz., a resolution) so as to keep it flat; the MTF on an infrared detector is made flat and the MTF in relation to a low pass filter in an electric circuit system is devised to prevent high frequency region noise in an analog-to-digital conversion from falling to a low frequency region by sampling.
Nevertheless, a conventional infrared observation system has a problem in that, based on an aberration of the optical system, scanning distortion and linearity of the scanning system, and an area of the infrared detector or the like, their transfer characteristics that are dependent on spatial frequency will deteriorate. Accordingly, when a signal that makes the resolution change at a necessary high speed is passed thereto, the frequency component in the high frequency region is attenuated to produce a signal in which the waveform is made round on the time axis (a signal such that the energy in a high frequency region is extracted therefrom), the amplitude is reduced, and thus a representation that is sharp as a picture image (a change or an edge cannot be discriminated) cannot be provided.
In the prior art, since frequency compensation in the whole system of the device is not taken into account for from the beginning of design, the design is effected by a cut-and-try method and directed to provide for the compensation only by a performance allowance such as NETD (noise equivalent target temperature difference). Therefore, a compensation circuit is too complex and thereby results in an ineffective and a halfway compensation because of lack of filter space. In the conventional device, the above compensation method or an equalizing method was not carried out because a noise that has no frequency characteristic is over-emphasized in the high frequency region after equalizing, to thereby increase the total noise, sensitivity is reduced, and accordingly, it is necessary to improve the performance required of the detector. Also, the NETD of the device, the sensitivity of which is endowed with more importance than easy viewing of the picture image, is considered an essential performance feature and evaluating and testing all systems of the device is generally performed visually. Finally, a quantitative design procedure for the compensation has not yet been found.
From the above, until the infrared energy from the target object is reflected on the displayed screen and enters the human eye, the design that makes the total frequency characteristics flat (viz., equalizing) by the mutual compensated design between the components of the system considered has not been set out, the high frequency region of transfer characteristics for the infrared observing system is always lowered in spite of consecutive labor, the integrated performance of the image ordinarily reflected does not harmonize therewith, the image does not have a strong appeal and the edge or the like is not sharp and it is very hard to view the picture image.
Therefore, in an infrared-ray observation device that, after an infrared-ray from the optical and scanning system is converted into an infrared-ray image signal by the infrared-ray detector, the high frequency component of the infrared-ray image signal is filtered by a low pass filter to cut-off a high frequency region noise and display the output on the display unit, and an object of the present invention is to provide an infrared observing system that, even if the sensitivity of the device, viz., NETD has more or less deteriorated, the frequency characteristic is rendered flat to thereby obtain a correct and obvious picture image in conformity with temperature information of an incoming infrared-ray.